.As a personal preference, I like the 7.62 X 39 round far more than the 5.56mm. Although the chart shows the "range" of the AR VS the AK, our army boys are picking up AK's and using them due to the fact the AK will penetrate at a few hundred yards and the AR won't!
I've heard some anecdotal stories about this happening, but little beyond. The x39 round looses it's "umpf" pretty quickly.
Looking at the plastic features of the AR, and the reliability built into the AK design, I personally believe the AK will outlast the AR by several years. Don't really understand where the people that built the chart believe otherwise. (I always believe what the internet says- NOT).
Looking at a few thousand dug up from caches, would probably change your mind. Rifles that last more than a couple of decades in completely harsh environs have usually been well cared for, even if they get very worn.
To me effective range should be the range at which a bullet does the job intended - not the distance where it will print on a paper target. I have a few SKS's, AK and LR-308. I did buy a couple of lowers for AR's, and will probably build at least one up into a rifle, but probably will build it into a 450 Bushmaster, 6.8 SPC, 458 Socom or similar. May build one up into 5.56 later - not sure. I do have a lot of high power military rifles - 6.5 X 55, 7.5 Swiss, 303, 8 mm, 7.62 X 54, 30-06 (Garands & US Enfield) and others. Also have .243, .308's and even a little M1 carbine (30 cal.)
I Just like a Rifle that can reach out and do what it is supposed to do every time!
Then get an 5.56 AR, at least you will have some effective range.